(1) Field of the Invention
This invention relates to a miniature axisymmetric streamline tensile (MAST) specimen that is to undergo tensile stress. This MAST specimen is axisymmetrical with transition regions (i.e., regions that transition from the constant diameter shoulder or grip regions near the end sections to the gauge section region) having variable curvature fillets and having a surface stress concentration factor (SSCF) close to unity.
(2) Description of the Prior Art
The Navy has a need to test piezoelectric ceramic materials in order to determine their performance in sonar transducers. The piezoelectric ceramic materials are typically grown as a single crystal which limits the size of the sample. These materials are very brittle and subject to cracking during use as a transducer. The Navy needs to know the useful life of these materials and the amount of stress that they can be subjected to while still being useful. It is also necessary to determine the electrical properties of the materials and how small flaws in the material affect these properties. No prior art test configuration properly gives this information for piezoelectric ceramics.
Tensile testing of ceramic materials using a standard tensile testing machine is not commonly performed because the tensile strength of ceramic materials is typically very sensitive to small cracks. These cracks are almost always present in normally sized specimens. In brittle materials, such as these ceramic materials, no energy is dissipated in plastic deformation ahead of the crack and the crack propagates easily. A bend test is more commonly used to determine the transverse rupture strength of a ceramic; however, this test does not give the true tensile strength of the material, and the ceramic is subject to failure at the points of load. Another problem is that properties determined using the bend test are not independent of the volume of material being tested.
The American Society for Testing and Materials (ASTM) is a large, not-for-profit, standards organization that provides a forum for producers, users, ultimate consumers, and those having a general interest to meet on common ground and write standards for materials, products, systems, and services. ASTM develops and publicizes voluntary consensus standards for materials, products, systems, and services. ASTM also publishes standard test methods, specifications, practices, guides, classifications, and terminology. ASTM's standards development activities encompass metals, paints, plastics, textiles, petroleum, construction, energy, the environment, consumer products, medical services and devices, computerized systems, electronics, and many other areas.
Several commonly used standardized tensile and compression specimen shapes can be found within the ASTM literature. Tension specimens are generally either flat or axisymmetric shaped. They are typically loaded for testing using wedge grips, collets, threaded ends or pinned ends. Typically, these existing specimen shapes are several inches or longer in length and include constant radius fillets that transition a grip region to a gauge section.
The use of constant curvature fillets, while reducing the complexity of specimen machining and costs, results in surface stress concentration factors (SSCF) ranging minimally from 1.10 to 1.20. A desired SSCF value is unity, 1.00. Furthermore, these specimens yield material strengths that are dependent upon specimen profiles.
With these specimens, the maximum stress may not occur within the gauge section where the applied stress field is assumed uniaxial. Rather, the maximum stress resides at a surface transition point where stress fields are neither uniaxial nor uniform, but rather biaxial and highly non-uniform. For materials that are brittle or lacking sufficient ductility, the surface transition points may become failure initiation regions, especially under dynamic fatigue loads. No suitable miniature axisymmetric standard tensile specimens were available that provide SSCFs close to unity.
The prior art discloses various testing specimens. One such prior art specimen is Van Winkle et al., U.S. Pat. No. 2,454,850, which is said to disclose a torsion specimen having a cylindrical gauge region.
Also known in the prior art is Scott et al., U.S. Pat. No. 4,606,230, which is said to disclose a tensile testing apparatus with a tensile specimen having a rectangular gauge section.
Also known are Pratt, U.S. Pat. No. 4,895,750, and Pratt, U.S. Pat. No. 5,078,843, which are said to disclose a carbon composite tensile test specimen for high temperature testing and a method of fabricating the same. The tensile test specimen has a central gauge section that appears to be curved and of constant dimension.
Also known is Hiyoshi, U.S. Publication No. 2002/0166386 A1, which is said to disclose a method and apparatus for measuring elongation in a contact-less manner capable of obtaining accurate measured value without attaching reference lines and capable of being automated wherein a test specimen has a straight, constant-width gauge section.
Also known is Oplinger et al., On the Streamline Specimen for Tension Testing of Composite Materials, Special Technical Testing Publication 864—American Society for Testing and Materials, pp. 541–542, Philadelphia, 1985, which is said to disclose the analogy between elastic stress fields and 2-D fluid flow through a reducer. Concerning the testing of fibrous composites, this publication teaches the use of a flat, streamlined specimen to reduce surface stress concentration factors to near unity. Wedge grips are used to hold the flat sample in the test apparatus.
Other devices and specimens are known for tensile and compression testing of various materials. These specimens, along with those above, have various shortcomings including having maximum stress not occurring within a gauge section, failure regions at surface transition points and surface stress concentration factors (SSCF) not at unity. The shortcomings of these specimens are addressed by the present invention.